Enable STACK's plot command to plot "discrete" and "parametric" plots.
Refactoring the plot command to deal with options.  Alt next must now be specified as [alt,"Time"] not as an equation.
Basic support for options in the plot command.  Not all options added, and some (e.g. axes) may not work in Maxima (never mind in STACK).  It is a start, and a significant improvement.
Merge branch 'master' of github.com:maths/moodle-qtype_stack